The polar regions are undergoing rapid climate change. There is a general amplification of global warming in the Arctic: surface air temperatures have warmed at approximately twice the global rate, although there are local variations. The average warming north of 60° N has been 1-2 °C since a temperature minimum in the 1960s and 1970s with the largest increase (c. 1 °C per decade) in winter and spring. Continental arctic land masses together with the Antarctic Peninsula are the most rapidly warming areas of the globe. Precipitation in the Arctic shows trends of a small increase over the past century (about 1% per decade), but the trends vary greatly from place to place and measurements are very uncertain. There are reductions in Arctic sea ice, river and lake ice in much of the sub-Arctic, and Arctic glaciers. Reduction in Arctic sea ice has occurred at a rate of 8.9% per decade for September relative to the 1979 values and there was an un-predicted extreme reduction in 2007. Permafrost has warmed. Although changes in the active layer depth have no general trend, in some sub-Arctic locations, discontinuous permafrost is rapidly disappearing and changes in permafrost are driving changes in hydrology and ecosystems. In Arctic Russia, ponds are drying in the continuous permafrost zone and waterlogging is occurring where there is discontinuous permafrost.
In Antarctica, temperature trends show considerable spatial variability: the Antarctic Peninsula shows significant warming over the last 50 years, whereas cooling has occurred around the Amundsen-Scott Station at the South Pole and in the Dry Valleys. Consequently, there is no continent-wide polar amplification of global change in Antarctica.
Current polar warming is leading to changes in species' ranges and abundance and a northward and upward extension of the sub-Arctic treelines. Forest is projected to displace considerable areas of tundra in some places. Species tend to relocate, as they have in the past, rather than adapt to new climate regimes. However, this process is likely to lead to the loss of some species: polar bears and other ice-dependent organisms are particularly at threat. In other areas, where rates of species relocation are slower than climate change, the incidence of pests, disease, and fire is likely to increase. Changes in vegetation, particularly a transition from grasses to shrubs, have been reported in the North American Arctic, and satellite imagery has indicated an increase in the 'normalized difference vegetation index' (a measure of photosyntheti-cally active biomass) over much of the Arctic. This index has increased by an average of about 10% for all tundra regions of North America, probably because of a longer growing season. However, such increases in productivity and changes in plant functional types have been shown experimentally to displace mosses and lichens that are now major components of Arctic vegetation.
In Antarctica, warming has caused major regional changes in terrestrial and marine ecosystems. The abundances of krill, Adelie, and Emperor penguins and Weddell seals have declined but the abundances of the only two native higher plants has increased. On continental Antarctica, climate change is affecting the vegetation composed of algae, lichens, and mosses. Introductions of alien species, facilitated by increased warming and increased human activity, are particular threats to southern ecosystems. Recent studies on sub-Antarctic islands have shown increases in the abundance of alien species and negative impacts on the local biota. In contrast, cooling has caused clear local impacts in the Dry Valleys where a 6-9% reduction in lake primary production and a 10% per year decline in soil invertebrates has occurred.
The responses of polar environments to climatic warming include feedbacks to the global climate system and other global impacts. Increased runoff from arctic rivers could affect the thermohaline circulation that redistributes the Earth's heat, thereby causing cooling in the North Atlantic and further warming in the tropics. Reductions in sea ice extent and snow cover together with a shift in vegetation from tundra to shrubs or forests are likely to reduce albedo (reflectivity of the surface) and lead to further warming despite the increased uptake of carbon dioxide by a more productive vegetation. Thawing permafrost is likely to release methane, a parti cularly powerful greenhouse gas, and evidence of this is already available from various arctic areas.
Not all impacts of climate warming in polar regions are disadvantageous to society: the reduction of sea ice in the Arctic is likely to lead to increased marine access to resources and new fisheries and reduced length of sea routes, while warming on land will probably lead to increased productivity and increased potential for forestry and agriculture.
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